Oxidative Water Treatment: The Track Ahead

Chemical oxidation has been applied in municipal water treatment for more than a century, initially for disinfection. In the early decades, chlorine disinfection was adopted in the fight against waterborne disease. However, the oxidative properties of chlorine had an unintended side effect, generation of potentially toxic disinfection byproducts (DBPs). To mitigate this issue, alternative disinfectants such as chlorine dioxide, chloramine, and ozone were employed, but they posed a different set of hazards. (2) Eventually, it was recognized that operators of drinking water treatment plants had to strike a balance between disinfection and DBP formation. Today, a multidisciplinary approach that combines tools from analytical chemistry, engineering, toxicology, and epidemiology is being employed to navigate the disinfection/DBP trade-off. (3) Chemical oxidation also has been applied for many decades to control chemical contaminants, initially for improvement of the aesthetic qualities of water. By the late 1970s, oxidants also were being used to treat synthetic organic compounds, termed micropollutants or trace organic contaminants. Chemical oxidation was employed first for abatement of nonpolar contaminants (chlorinated solvents, pesticides, and fuel additives). As analytical methods improved and water utilities pursued enhanced wastewater treatment and potable water reuse, treatment was extended to polar micropollutants, including pharmaceuticals, personal care products, endocrine disruptors, and industrial chemicals. (4) As the variety of chemicals requiring treatment expanded, a second trade-off became evident. Selective oxidants, such as permanganate, chlorine dioxide, and chlorine, are advantageous for treatment of waters containing mainly one compound class (e.g., phenolic compounds) because such contaminants can be treated with a high oxidant use efficiency. Advanced oxidation processes (AOPs) that employ a combination of oxidants (e.g., O3 with H2O2) or use ultraviolet light (e.g., UV with H2O2 or UV with chlorine) generate hydroxyl radical (•OH) and other short-lived oxidants. These reactive species oxidize a wider range of chemicals and are thus attractive for the treatment of water that contains multiple contaminants of concern. Broad reactivity usually is accompanied by low oxidant use efficiency because a substantial fraction of the oxidant reacts with nontarget solutes, like dissolved organic matter. Ozone is a special case because it can achieve high selectivity for some compounds through direct reactions while simultaneously producing enough •OH to abate compounds that lack functional groups that are susceptible to direct oxidation. Thus, the users of oxidative treatment technologies must navigate the trade-off between selectivity and efficiency. The disappearance of target compounds during oxidation is often accompanied by a decrease in their biological effects. However, ensuring that none of the transformation products produced dur